Preheat oven to 180C/160C Fan/Gas 4.

Grease or line a baking tin (I used a round baking tin with a diameter of 21cm)

Cream the softened butter and sugar together in a bowl.

Add the lemon zest, lemon juice (if using) and sift the flour in and mix together.

Use your hands to knead it together to form a dough.

Transfer dough to a greased baking tray, pressing down until it reaches the edges. Use the back of a spoon or spatula if necessary to smooth it over.

Using a fork score some holes and then, with a knife, slice the shortbread up into portions.

Put the shortbread tin in preheated oven and cook for 15 to 20 minutes or until the edges of the shortbread start to turn golden brown in colour and the dough is cooked through.

Remove the shortbread from the oven and before it cools down, slice up the portions again.

Optionally sprinkle some more caster sugar over the top.